1. Field of the Invention (Technical Field):
The present invention relates to methods for making back-contact solar cells, and particularly emitter wrap through (EWT) solar cells with conductive vias, and solar cells made by such methods.
2. Description of Related Art:
Note that the following discussion refers to a number of publications by author(s) and year of publication. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The solar cell design in widespread use today has a p/n junction formed near the front surface (that surface which receives the light) which creates an electron flow as light energy is absorbed in the cell. The conventional cell design has one set of electrical contacts on the front side of the cell, and a second set of electrical contacts on the rear side of the solar cell. In a typical photovoltaic module these individual solar cells are interconnected electrically in series to increase the voltage. This interconnection is typically accomplished by soldering a conductive ribbon from the front side of one solar cell to the rear side of an adjacent solar cell.
Back-contact silicon solar cells have several advantages compared to conventional silicon solar cells. The first advantage is that back-contact cells have a higher conversion efficiency due to reduced or eliminated contact obscuration losses (sunlight reflected from contact grid is unavailable to be converted into electricity). The second advantage is that assembly of back-contact cells into electrical circuits is easier, and therefore cheaper, because both polarity contacts are on the same surface. As an example, significant cost savings compared to present photovoltaic module assembly can be achieved with back-contact cells by encapsulating the photovoltaic module and the solar cell electrical circuit in a single step. The last advantage of a back-contact cell is better aesthetics through a more uniform appearance. Aesthetics is important for some applications, such as building-integrated photovoltaic systems and photovoltaic sunroofs for automobiles.
FIG. 1 provides an illustration of a generic back-contact solar cell 10. The silicon substrate 12 may be n-conductivity type or p-conductivity type. One of the heavily doped emitters, such as p++ doped emitter 18 or n++ doped emitter 16 may be omitted in some designs. Alternatively, the heavily doped emitters 16, 18 may directly contact each other on the rear surface in other designs. Rear-surface passivation 14 helps reduce loss of photogenerated carriers at the rear surface, and helps reduce electrical losses due to shunt currents at undoped surfaces between the metal contacts 20.
There are several approaches for making a back-contact silicon solar cell. These approaches include metallization wrap around (MWA), metallization wrap through (MWT), emitter wrap through (EWT), and back-junction structures. MWA and MWT have metal current collection grids on the front surface. These grids are, respectively, wrapped around the edge or through holes to the back surface in order to make a back-contact cell. The unique feature of EWT cells, in comparison to MWT and MWA cells, is that there is no metal coverage on the front side of the cell, which means that none of the light impinging on the cell is blocked, resulting in higher efficiencies. The EWT cell wraps the current-collection junction (“emitter”) from the front surface to the rear surface through doped conductive channels in the silicon wafer. “Emitter” refers to a heavily doped region in a semiconductor device. Such conductive channels can be produced by, for example, drilling holes in the silicon substrate with a laser and subsequently forming the emitter inside the holes at the same time as forming the emitter on front and rear surfaces. Back-junction cells have both the negative and positive polarity collection junctions on the rear surface of the solar cell. Because most of the light is absorbed—and therefore also most of the carriers are photogenerated—near the front surface, back-junction cells require very high material quality so that carriers have sufficient time to diffuse from the front to the rear surface with the collection junctions on the rear surface. In comparison, the EWT cell maintains a current collection junction on the front surface, which is advantageous for high current collection efficiency. The EWT cell is disclosed in U.S. Pat. No. 5,468,652, Method Of Making A Back Contacted Solar Cell, to James M. Gee, incorporated here in full. The various other back-contact cell designs have also been discussed in numerous technical publications.
In addition to U.S. Pat. No. 5,468,652, two other U.S. patents on which Gee is a co-inventor disclose methods of module assembly and lamination using back-contact solar cells, U.S. Pat. No. 5,951,786, Laminated Photovoltaic Modules Using Back-Contact Solar Cells, and U.S. Pat. No. 5,972,732, Method of Monolithic Module Assembly. Both patents disclose methods and aspects that may be employed with the invention disclosed herein, and are incorporated by reference as if set forth in full. U.S. Pat. No. 6,384,316, Solar Cell and Process of Manufacturing the Same, discloses an alternative back-contact cell design, but employing MWT, wherein the holes or vias are spaced comparatively far apart, with metal contacts on the front surface to help conduct current to the rear surface, and further in which the holes are lined with metal.
Under certain conditions, EWT cells with gas dopant diffused vias exhibit high series resistance associated with conduction through the vias. [J. M. Gee, M. E. Buck, W. K. Schubert, and P. A. Basore, Progress on the Emitter Wrap-Through Silicon Solar Cell, 12th European Photovoltaic Solar Energy Conference, Amsterdam, The Netherlands, April 1994]; Gee J M, Smith D D, Garrett S E, Bode M D, Jimeno J C: Back-Contact Crystalline-Silicon Solar Cells and Modules. NCPV Program Review Meeting, 8–11 Sep. 1998, Denver, Colo. One approach to addressing this problem is to fill vias with metal, such as plated metal. However, this approach adds significant complexity to the manufacturing process and is accordingly more expensive. Another approach is to increase the density of vias such that an acceptable series resistance is attained. However, this also adds complexity and cost. A preferred approach is to dope the holes more heavily than the surfaces, as long as the process maintains simplicity and low cost. At least some data suggests that conventional gas diffusion, such as gas phase diffusion using liquid POCl3, results in less diffusion within the hole than on horizontal or planar surfaces, possibly because dopant gasses do not penetrate to the interior of the hole as effectively as to an exposed surface. However, other data found that the hole conductivity was high and consistent with interior doping similar to the exposed surfaces [D. D. Smith, J. M. Gee, M. D. Bode, J. C. Jimeno, Circuit modeling of the emitter-wrap-through solar cell, IEEE Trans. on Electron Devices, Vol. 46, 1993 (1999)].
A critical issue for any back-contact silicon solar cell is developing a low-cost process sequence that also electrically isolates the negative and positive polarity grids and junctions. The technical issue includes patterning of the doped layers (if present), passivation of the surface between the negative and positive contact regions, and application of the negative and positive conductivity contacts.